With the development of technology in recent years, various light measurement devices that employ light interference have been developed. There are various types of devices based on interferometers that use the interference of light, and these devices are used according to the purpose. Fizeau interferometers and Mach-Zehnder interferometers have been used for the purpose of performing high-precision planar measurement and spherical measurement by using a simple construction.
As an example of an interferometer, a laser interference device that uses a Fizeau interferometer is disclosed in Patent Document 1. Referring to FIGS. 1 and 2 of Patent Document 1, the laser interference device both irradiates the measurement object and irradiates a reference master standard with laser light by way of a beam splitter and collimator lens and condenses the reflected light from the reference surface of the reference master standard and the detection surface of the measurement object upon a translucent screen by way of collimator lens and beam splitter. A light-dot imaging lens and interference fringe imaging lens are arranged on the optical path behind the screen so as to allow exchange and a solid-state imaging device is provided at the focal point position of the light-dot imaging lens. The light dot image on the screen is captured using the light-dot imaging lens, and the interference fringe that passes through the screen is observed using the interference fringe imaging lens.
The technology of Optical Coherence Tomography (OCT) is widely utilized in the field of, for example, funduscopy. OCT is a technology that allows non-invasive imaging at a spatial resolution of approximately 10 μm of a tomographic image having a depth of 1-2 mm of biological tissue. OCT takes as a basis a Michelson interferometer that uses low-coherence light. OCT irradiates light upon bio-tissue, selectively detects the straight advancing light component that is reflected from inside the tissue, and based on this detection, constructs a two-dimensional or three-dimensional tomographic image.
For example, Non-Patent Document 1 describes a schematic diagram that shows the basic configuration of OCT. FIG. 1 is borrowed from FIG. 5.1 of Non-Patent Document 1. The basic configuration of OCT includes a light source, a reference light mirror, a beam splitter, and a photodetector. The measurement object is, for example, an organism.
The light source emits low-coherence light of the near-infrared region. Low-coherence light refers to light having extremely low temporal coherence. Further, the light source is made up of Super Luminescent Diodes (SLDs).
As shown in FIG. 1, the light source emits low-coherence light toward the beam splitter. At this time, the low-coherence light is assumed to have a center wavelength of 850 nm and a full width at half maximum of spectrum of 20 nm.
The light from the light source is split into two by the beam splitter. One beam of the light that was split into two by the beam splitter is directed toward the reference mirror, and after being reflected by the reference mirror, is again returned to the beam splitter as reference light ER. The other beam of the light that was split into two by the beam splitter is irradiated toward the measurement object as measurement light. The innumerable beams of reflected light (signal light) ES (as examples, reflected light ESA and ESB) from the surface and interior of the measurement object return to the beam splitter. Half of each of the beams of reference light ER and reflected light ES that have returned back to the beam splitter converge on the same optical path and interfere with each other before the photodetector.
As shown in FIG. 1, particular reflection surfaces of the surface and interior of the measurement object along the direction of propagation of the signal light are here taken as A and B. Taking the beam splitter as a point of reference, if reflection surface A inside the measurement object and position 1 of the reference light mirror are optically equidistant, the time periods of the sine wave oscillation of reference light ER1 and reflected light ESA overlap and the two undergo interference. As a result, the photodetector obtains interference light of reference light ER1 and reflected light ESA. Next, in order to obtain the interference light of reference light ER2 and reflected light ESB, the reference light mirror is moved in a direction away from the beam splitter and up to position 2 that is equidistant with reflection point B. In this way, the photodetector can obtain the intensity distribution of reflected light in the direction of the optical axis by continuously moving the reference mirror to detect interference light.
As described hereinabove, there are various light measurement devices that use the interference of light, and a variety of schemes have been implemented and forms adopted according to the object of the devices.
However, in the technology described in Patent Document 1, the light-dot imaging lens used when adjusting the optical axis and the interference fringe imaging lens for observing an interference fringe must be arranged on the optical path so as to allow exchange, and this requirement entails such problems as an increase in complexity of the configuration and an increase in the number of parts. In addition, the operations of optical axis adjustment and observation of interference fringe are accompanied by the operation of each time exchanging these lenses, and as a result, there is the problem that rapid measurements cannot be realized when specimens are frequently exchanged and measured.
One example of a technology for solving these problems is disclosed in Patent Document 2. As shown in FIG. 1 of Patent Document 2, a technology is disclosed in Patent Document 2 that relates to an interferometer that allows simplification of the configuration and that allows interference fringe observation to be rapidly carried out after optical axis adjustment. According to the invention disclosed in Patent Document 2, when the optical axis of the reference surface of the reference lens coincides with the optical axis of the inspection surface of a specimen, the interference light beam passes through a half-mirror surface and the opening of a diaphragm plate and is condensed by a condensing optical system, and then an image of the interference fringe is captured by an imaging unit.
On the other hand, when the optical axis of the reference surface of the reference lens does not coincide with the optical axis of the inspection surface of a specimen, the optical axis of the reference light that is reflected by the reference surface diverges from that of the measurement light that is reflected by the inspection surface, and at least one of the beams reaches the reflection surface of the diaphragm plate to produce a component that is reflected on the half-mirror surface side. This reflected light component on the reflecting surface reaches the half-mirror surface and a portion of this reflected light passes through the opening of the diaphragm plate and is condensed by the condensing optical system. Since the condensing optical system is provided so that the reflecting surface of the diaphragm plate and the imaging surface of the imaging unit are conjugated by the way of the half-mirror surface, the image of the reflected light component on the reflecting surface of the diaphragm plate can be captured by the imaging unit. As a result, the positional shift of the reference light and the measurement light with respect to the opening of the diaphragm plate can be observed through the imaging unit. Accordingly, the observation during the optical axis adjustment and observation of the interference fringe can be carried out based on the image that was captured by the imaging unit without exchanging the condensing optical system.
In addition, an image processing device and interferometer measurement system that can simplify the device configuration for displaying the image of an interferometer and improve the workability of interference fringe measurement are disclosed in Patent Document 3.
Referring to FIG. 1 of Patent Document 3, the image processing device has: a connection terminal unit that connects an alignment camera that captures images for alignment with an interference fringe camera that captures an interference fringe image and that transmits image data that are transmitted from the cameras; a camera changeover switch that selects, of the image data that have been transmitted, an alignment image or an interference fringe image; a display unit that displays images according to the selected image data; an image processing unit that analyzes image data of an interference fringe image; and a device control unit that, when image data of an interference fringe image has been selected by the camera changeover switch, displays the analysis results that have been analyzed by the image processing unit together with interference fringe image on the display unit.